Heat 1 tbsp oil in a large frying pan or wok, and add the diced onion, aubergine, pepper, courgette and mushrooms. Cook over a medium heat for around 10 minutes, stirring regularly, until the vegetables are soft and just starting to turn golden. Add the minced garlic and dried oregano, and cook for a couple more minutes.
Remove the vegetables from the pan, and set aside. Add the remaining 1 tbsp oil to the same pan, and add the arborio rice (alternatively, if you prefer, you can use a separate pan for the rice and cook both simultaneously).
Cook the rice in the oil for a minute or two over a medium-low heat, stirring constantly, then add a little of the vegetable stock. Stirring regularly, allow the rice to absorb most of the excess liquid. Continue adding the stock a little at a time, allowing any excess to be absorbed each time before adding more. When you've used all the stock and the rice is soft, add the tomato puree, balsamic vinegar, and basil pesto, and mix well. Return the cooked vegetables to the pan, and mix well to combine.
Serve topped with fresh basil if desired.